Support ThinArc for other dynamically sized types

[thomcc]

A ThinArcStr would be pretty easy to support (would just wrap a ThinArc<H, u8> -- you can already do this by using triomphe).

The use case I care more about would be a way of having a 1-pointer Arc (pretty useful for FFI stuff)... but I'm not sure how possible this is though, given that we don't have a way to get a trait object vtable pointer...

[NobodyXu]

Even if we can obtain that, the layout of that vtable can still change though.

So maybe just generate an explicit vtable for the types you care?

[Manishearth]

It probably can be done with the new (unstable) pointer metadata APIs

[NobodyXu]

But ASAIK it doesn't provide any API for retrieving the vtable and call one of these methods.

[Manishearth]

you can reconstruct the trait object as an &dyn on the rust side

[NobodyXu]

Sorry I misunderstood the topic.
I was thinking about calling rust dyn trait from C/C++.

If you are talking about passing the pointer to C/C++ then get it back, I think to_raw_parts and from_raw_parts can be used to archive this, though this are available only on nightly channel.